This invention relates to a mechanical variable capacitor and, more particularly, to a variable capacitor small in size and high in performance and quality, for use on a densely integrated electric circuit, and to a method for manufacturing the same.
A variable capacitor is utilized as a microfabricated mechanical switch using a variable capacitance structure. FIGS. 1A-1C show a microwave switch introduced in IEEE MTT-S Digest 1999, pp. 1923-1926.
A gold contact 26 is provided, through an insulation layer 25, at a tip lower part of a silicon cantilever 21. On a surface opposed to the contact 26, there are provided a circuit terminal 27 for forming a close circuit upon contacted with the contact 26, and a driving electrode 28 for providing an electrostatic force to the contact 26 to thereby deflect the silicon cantilever 21. The silicon cantilever 21 has a length of approximately 200 xcexcm, a width of approximately 20 xcexcm and a thickness of approximately 2.5 xcexcm. The contact 26 and the circuit terminal 27 has a gap set at 10 xcexcm or less. By applying a voltage of 50 V or higher to the driving electrode 28, the beam 21 deflects to place the contact 26 into contact with the circuit terminal 27, thereby closing the contact.
However, because the voltage required for closing the contact is as high as 50 V or greater, there is a need to mount an exclusive booster circuit, posing hindrance against miniaturizing the switch element. Also, as the area is broader in the pad formed at the tip of the silicon cantilever 21, the more the viscosity resistance of ambient air is undergone during vertical driving, to decrease the operation speed. High speed switching on a several is order is difficult to attain.
FIG. 2 is a conventional beam structure that enables a low voltage driving and switching speed on an order of several xcexcs. A beam 31 has a size of a width W=2 xcexcm, a thickness t=2 xcexcm and a length L=500 xcexcm. On the substrate 34, an electrode 32 formed, on its surface, an insulation layer 35 having a film thickness of 0.01 xcexcm is arranged through an air gap of 0.6 xcexcm to the beam 31. In case a voltage V is applied to between the beam 31 and the electrode 32, the beam 31 deflects in xe2x88x92z direction due to an electrostatic force. At a pull-in voltage or higher, the electrostatic force becomes greater than a restoration force of the beam 31 thereby increasing the force. Accordingly, the beam 31 is immediately attracted onto the insulation layer 35. If the voltage is raised furthermore, the beam 31 gradually increases the capacitance with the electrode 32 while increasing the contact area with the insulation layer 35.
In this manner, the beam 31 can be weakened in springiness by increasing the length of the beam 31. Also, by narrowing the width of the beam 31 and thereby reducing the viscosity resistance of air, it is possible to attain low-voltage driving and switching speed on an order of several xcexcs. When the beam 31 uses, as material, aluminum having a Young""s modulus 77 GPa, the pull-in voltage is 0.25 V in the case the beam 31 is provided as a cantilever and 1.72 V when it is supported at both ends.
However, such an elongate beam geometry involves conspicuous problems of (1) residual stress, (2) thermal expansion, and (3) stiction.
The first problem of residual stress is mentioned. In fabricating a fine beam, used is a thin-film structure using a semiconductor process, a thin-rolled material junction structure, or the like. In any case, the residual stress within the beam is problematic. Such residual stress includes two kinds, i.e. one is compressive/tensile stress to act in a beam lengthwise direction, and the other is stress gradient along a beam thickness direction.
For example, in case the beam of FIG. 2 is assumably a beam supported at both ends, when an excessive compressive stress remains in x and y directions in the figure, the stress release in the y direction does not cause a substantial change in the beam geometry. However, concerning the x direction in which the beam end surface is bound, buckling is caused in order to release stress. Thus, the beam deflects irrespectively of applying an electrostatic force.
Conversely, where tensile stress remains, the beam 31 apparently has no change. However, as the residual tensile stress increases, pull-in voltage increases to conspicuously change the beam driving characteristic. Namely, it is ideal to manufacture a beam with a residual stress of zero. However, unless internal stress is accurately controlled to a predetermined value in the beam manufacture process, variation is incurred in buckling or pull-in voltage, deteriorating element quality.
On the other hand, because this kind of stress on a cantilever is to be released, there is no occurrence of buckling or pull-in voltage variation. However, when the beam 31 is a cantilever, a stress gradient if exists in the z direction or in the beam thickness direction results in upward warps in the beam due to stress release. For example, in case a plus stress gradient 2 MPa/xcexcm exists along the z direction within the beam, the beam at its tip warp up 2 xcexcm. Unless the stress gradient value can be accurately controlled to a predetermined value in the beam manufacture process, there occurs variation in warp degree whereby it becomes impossible to suppress the capacitance-decrease variation and pull-in voltage increase variation due to increase in the distance between the beam 31 and the electrode 32. For example, pull-in voltage is 0.25 V in the case the stress gradient is zero in the absence of a warp, whereas pull-in voltage increases up to 1.2 V in the state of warping up 2 xcexcm at the tip.
It is quite difficult to control, in the manufacture process of the beam, the compressive/tensile strength in the lengthwise direction and the stress gradient in the thickness direction. Although there is xe2x80x9cannealxe2x80x9d as a stress relaxing method in the manufacture process, this process is to expose a device to an elevated temperature which temperature has an effect upon the device structuring members other than the beam. For example, in case the sacrificial material or the like, temporarily provided beneath the beam and finally etched away in order to make an electrode metal or beam in a bridge structure, is exposed to an elevated temperature, the material characteristic thereof changes. For this reason, because the element cannot be exposed to a high temperature, it is impossible to completely remove stress.
In the second thermal expansion problem, the beam causes therein thermal expansion in a lengthwise direction due to temperature rise around the element. In the both-ends-supported beam structure bound at both ends, the beam causes buckling to deflect irrespectively of electrostatic force application.
Next, the third stiction problem is mentioned. FIG. 3 represents a relationship between a voltage and a capacitance in the case residual stress is suppressed nearly zero on a structure that the beam 31 of FIG. 2 is made in a both-end-supported type. In case voltage is applied, pull-in takes place at 1.72 V. In case a voltage equal to or greater than that is applied, the beam 31 and the electrode 32 go into contact through the insulation layer 35, increasing the contact area and hence the capacitance. Conversely, in case voltage is lowered, when the voltage is lowered down to 0.64 V, the contact between the beam 31 and the electrode 32 is gradually released. This is because of weak springiness, or spring restoration force, of the beam 31. This means that, when the voltage is returned to 0 V, in case there exists an adsorbing force through the ambient water molecules, an adsorbing force due to residual charge or a van der Waals force in the contact area, the beam 31 cannot return to the initial state with high possibility. In order to avoid this, there requires a complicated structure, including a mechanism to forcibly drive the beam 31 in a direction separated from the electrode 32, e.g. newly providing an electrode for pulling back the upper surface of the beam 31 of the FIG. 2 by an electrostatic force.
There is a structure described in U.S. Pat. No. 5,818,683, as a prior art of variable capacitance the stiction problem is avoided. In this structure, the electrode opposed to the cantilever is divided as a driving electrode close to the root and a detection electrode positioned close to the beam tip. It is only the driving electrode that contributes to deflecting the beam on an electrostatic force. The slight deflection amount of the beam is controlled within a voltage range that pull-in-does not take place between the beam and the driving electrode. Utilizing a beam displacement at around the beam tip magnified on that principle, a capacitance change to the detection electrode is obtained.
In this structure, the problem of stiction is avoided because of control not to cause contact between the beam and the electrode. However, in also this structure, unless the foregoing stress gradient in beam thickness direction can be controlled in the manufacture process, variation occurs in warping up at the beam tip and hence variation in the capacitance to the detection electrode. Also, warp up decreases the capacitance, making it difficult to obtain a large variable capacitance change.
It is an object of the present invention to solve the foregoing problem and provide a variable capacitor simple in structure and high in quality, in a mechanical variable capacitor that low-voltage/high-speed driving is possible.
A variable capacitor of the present invention comprises: a beam having flexibility; and an electrode provided close to the beam in order to form a capacitor with the beam; whereby, by applying a voltage to between the beam and the electrode to thereby deflect the beam by an electrostatic force, a capacitance between the both is changed.
Meanwhile, the deflected beam and the electrode are placed into contact through an insulation layer formed on a surface of at least one thereof, to change a contact area thereof, thereby changing the capacitance.
Also, a recess is provided in the insulation layer such that a part of the electrode is lower than a height of a surface of another electrode. By applying a voltage to between the beam and another electrode and pulling the beam in the recess, the beam is separated from adhesion to the electrode, thereby eliminating stiction.
Also, the electrode for deflection by an electrostatic force is divided into plurality in the number. By providing the respective ones with a function to support the beam tip, a function as an alternating current signal line, and a function to eliminate stiction, the quality or performance deterioration due to residual stress, thermal expansion or stiction can be suppressed by a simple structure.